In the related art of light-emitting devices including excitation light sources and phosphors that convert wavelengths of at least part of light from the excitation light sources, white-light-emitting devices including blue-light-emitting diodes in combination with yellow phosphors have been put to practical use for various illumination applications. Typical examples thereof include devices described in Patent Documents 1, 2, and 3. An example of phosphors particularly used in these white-light-emitting devices is a cerium-activated yttrium-aluminum-garnet phosphor represented by a general formula(Y,Gd)3(Al,Ga)5O12:Ce3+.
However, white-light-emitting devices including blue-light-emitting diodes in combination with yttrium-aluminum-garnet phosphors emit bluish white light because of the lack of a red component, thus disadvantageously resulting in low color rendering properties.
Accordingly, the development of a white-light-emitting device in which the red component, in which light emitted from the white-light-emitting device including the yttrium-aluminum-garnet phosphor is lacking, is supplemented with another red phosphor has been studied. Such a light-emitting device is disclosed in, for example, Patent Document 4. Also in the light-emitting device disclosed in Patent Document 4 or the like, however, problems regarding the improvement of color rendering properties have yet to be solved. Thus, a light-emitting device in which the above problems are solved has been required. The red phosphor described in Patent Document 4 contains cadmium and thus is disadvantageous from the viewpoint of environmental pollution.
Patent Document 5 discloses a europium- or cerium-activated phosphor represented by a general formula Mx(SiOn)y (wherein M represents an alkaline-earth-metal element, i.e., Mg, Ca, Sr, Ba, or Ra). However, the document only discloses that as a specific example, Ba3SiO5:Eu2+ has an emission peak of 590 nm. (Ba1−aSra)3SiO5:Eu2+ is also exemplified, but the specific value of a is not described.
Non-Patent Document 1 discloses the crystal structure of a Ba3SiO5:Eu2+ phosphor. Non-Patent Document 2 discloses that the Ba3SiO5:Eu2+ phosphor is excited by an InGaN semiconductor laser having a wavelength of 405 nm. Non-Patent Document 3 discloses a Sr3SiO5:Eu2+ phosphor and discloses when Sr/Si ratios are 3/0.8, 3/0.9, 3/1, and 3/1.1, the emission peak wavelengths are 559 nm, 564 nm, 568 nm, and 570 nm, respectively.
Non-Patent Document 4 was published after the first filing date of the present invention and discloses that in (Ba1−xSrx)3SiO5:Eu2+, the partial occupation of the Sr sites by Ba allows the wavelength of light emitted from Eu2+ to shift to longer wavelengths to reduce the emission intensity. That is, the emission intensity of the (Ba1−xSrx)3SiO5:Eu2+ phosphor is lower than that of Sr3SiO5:Eu2+. Furthermore, the document describes a reduction in luminance by increasing the firing temperature of Sr3SiO5:Eu2+ from 1,250° C. to 1,350° C. Thus, Non-Patent Document 4 does not suggest the possibility of improvement in the emission properties of the (Ba1−xSrx)3SiO5:Eu2+ phosphor.    Patent Document 1: Japanese Patent No. 2900928    Patent Document 2: Japanese Patent No. 2927279    Patent Document 3: Japanese Patent No. 3364229    Patent Document 4: Japanese Unexamined Patent Application Publication No. H10-163535    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2005-68269    Non-Patent Document 1: Mitsuo Yamaga and four other persons, “Physical Review B”, 2005, vol. 71, pp. 205102-1 to 7    Non-Patent Document 2: Satoshi Yasuda and three other persons, “Proceedings of the 51st Spring Meeting of the Japan Society of Applied Physics”, p. 1607    Non-Patent Document 3: Joung Kyu Park and four other persons, “Applied Physics Letters”, 2004, vol. 84, pp. 1647    Non-Patent Document 4: Ho Seong Jang and two other persons, “Proceedings of the 12th International Display Workshops in Conjunction with Asia, display 2005 volume 1”, pp. 539-542
Hitherto, as described above, phosphors which are mainly composed of alkaline-earth metal silicates and which emit yellow to red light and light-emitting devices including the phosphors have been known.
However, the luminous efficiency thereof is insufficient. Phosphors and light-emitting devices having higher luminous efficiency have been required. Furthermore, the temperature of a phosphor incorporated in a light-emitting device has been known to rise to about 100° C. to 200° C. Thus, a phosphor and a light-emitting device in which luminous efficiency is not reduced even when the temperature rises have been required.